U.S. patent number 5,003,815 [Application Number 07/424,377] was granted by the patent office on 1991-04-02 for atomic photo-absorption force microscope.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Yves Martin, Hemantha K. Wickramasinghe.
United States Patent |
5,003,815 |
Martin , et al. |
April 2, 1991 |
Atomic photo-absorption force microscope
Abstract
An Atomic Photo-Absorption Force Microscope 1 includes an Atomic
Force Microscope 10 and a radiation source 20 having an output
radiation 22 wavelength selected to be preferentially absorbed by
atoms or molecules associated with a sample surface 24a under
investigation. Absorption of the radiation raises at least one
outer shell electron to a higher energy level, resulting in an
increase in radius of the atom or molecule. A tip 12 coupled
through a lever 14 to the Atomic Force Microscope 10 is scanned
over the surface and operates in conjunction with a laser
heterodyne interferometer 18 to directly measure the resulting
atomic or molecular increase of size, thereby detecting both the
presence and location of the atoms or molecules under
investingation. Operation in an a.c. mode by chopping the incident
radiation 22 and measuring the corresponding a.c. induced tip
movement beneficially increases the sensitivity of the technique,
particularly if the a.c. frequency is chosen at a resonance of the
tip-lever combination.
Inventors: |
Martin; Yves (Briarcliff Manor,
NY), Wickramasinghe; Hemantha K. (Chappaqua, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23682409 |
Appl.
No.: |
07/424,377 |
Filed: |
October 20, 1989 |
Current U.S.
Class: |
73/105; 850/33;
250/305; 374/6; 250/352; 977/863; 977/868 |
Current CPC
Class: |
B82Y
35/00 (20130101); G01Q 30/02 (20130101); G01Q
60/38 (20130101); Y10S 977/868 (20130101); Y10S
977/863 (20130101) |
Current International
Class: |
G01N
23/00 (20060101); G01B 15/00 (20060101); G01B
11/30 (20060101); G01N 21/00 (20060101); H01J
37/26 (20060101); G01B 21/30 (20060101); G01N
21/31 (20060101); G01B 015/00 (); G01B
011/30 () |
Field of
Search: |
;73/105,104
;250/305,336.1,393,395,526,216,338.1,341,352 ;374/6,45,7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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113844 |
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Jul 1983 |
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JP |
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31112 |
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Feb 1985 |
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JP |
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47601 |
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Feb 1988 |
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JP |
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30413 |
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Dec 1988 |
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JP |
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113643 |
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May 1989 |
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JP |
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1427180 |
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Sep 1988 |
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SU |
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Other References
"IBM Images Surfaces by Electron Tunneling"; Science, vol. 220; 1
Apr. 1983; pp. 43-44, Arthur L. Robinson. .
"Photothermal Modulation of the Gap Distance in Scanning Tunneling
Microscopy" Appl. Phys. Lett. 49(3) Jul. 21, 1986, Amer et al.
.
"Atomic Force Microoscope-Force Mapping and Profiling on a Sub
100-.ANG. Scale" J. Appl. Phys. 61(10), 15 May 1987, pp. 4723-4729,
Martin et al. .
"High-Resolution Capacitance Measurement and Potentiometry by Force
Microscopy" Appl. Phys. Lett. 52(13), pp. 1103-1105, Mar. '88,
Martin et al. .
"Tunneling Spectroscopy" 1988 Elsevier Science Publishers B. V.,
pp. 141-156, Nelissen et al. .
"Atomic Force Microscope" Physical Review Letters, vol. 56, No. 9,
3 Mar. 1986, pp. 930-933, Binning et al..
|
Primary Examiner: Noland; Tom
Attorney, Agent or Firm: Perman & Green
Claims
Having thus described our invention, what we claim as new, and
desire to secure by Letters Patent is:
1. Spectroscopic apparatus comprising:
means for illuminating a surface of a sample with radiation having
a characteristic wavelength selected for being absorbed by atoms or
molecules of interest such that the atoms or molecules of interest
increase in radius;
means for translating a probe tip proximal to the surface of the
sample, the probe tip being mounted such that it experiences a
detectable movement in response to being positioned near to an atom
or molecule of increased radius; and
means for detecting the movement of said probe tip for indicating
the presence of the atoms or molecules of interest.
2. Spectroscopic apparatus as set forth in claim 1 wherein said
means for translating and said means for detecting comprise an
Atomic Force Microscope.
3. Spectroscopic apparatus as set forth in claim 1 wherein said
means for detecting is comprised of a laser heterodyne
interferometer.
4. Spectroscopic apparatus as set forth in claim 1 wherein said
probe tip has at least two characteristic resonant frequencies and
wherein said means for illuminating further comprises means for
chopping the radiation at a frequency that is a function of a first
one of the characteristic resonant frequencies.
5. Spectroscopic apparatus as set forth in claim 4 and further
comprising means coupled to said probe tip for oscillating said
probe tip at a second one of the characteristic resonant
frequencies.
6. A method of performing spectroscopy at atomic scales comprising
the steps of:
illuminating a sample with radiation having a characteristic
wavelength selected for being absorbed by atoms or molecules of
interest such that at least one electron associated with the atom
or molecule of interest is raised to a higher energy level,
resulting in an increase in a radius of the atom or molecule;
translating a probe tip proximal to a surface of the sample, the
probe tip being mounted such that it experiences a detectable
movement in response to being positioned near to an atom or
molecule of increased radius; and
detecting the movement of the probe tip for indicating the presence
of the atoms or molecules of interest.
7. A method as set forth in claim 6 wherein the probe tip has at
least two characteristic resonant frequencies and wherein the step
of illuminating includes a step of chopping the radiation at a
frequency substantially equal to a first one of the characteristic
resonant frequencies.
8. A method as set forth in claim 7 wherein the step of translating
includes a step of oscillating the probe tip perpendicularly to the
surface at a second one of the characteristic resonant
frequencies.
9. A method as set forth in claim 7 and including a step of
cancelling spurious vibration of the sample due to bulk thermal
expansion of the sample, the step of cancelling including a step of
illuminating the sample with chopped radiation having a second
wavelength that differs from the characteristic wavelength and that
is chopped out of phase therewith.
10. A method as set forth in claim 6 and including a step of
cancelling spurious vibration of the sample, the step of cancelling
including a step of applying an a.c. voltage between the probe tip
and the sample.
11. Apparatus for analyzing a sample comprising:
means for illuminating a surface of a sample with a chopped
radiation beam having a characteristic wavelength selected for
being absorbed by atoms or molecules of interest such that at least
one electron associated therewith is raised to a higher energy
level resulting in an increase in a radius of the atom or molecule;
and
Atomic Force Microscope means comprising:
means for translating a tip closely adjacent to the surface of the
sample and means for maintaining the tip at a substantially
constant distance from the surface of the sample, the tip
experiencing a detectable movement in response to being positioned
near to an atom or molecule of increased radius; and
means optically coupled to the tip for detecting the movement of
the tip for indicating the presence of the atoms or molecules of
interest.
12. Apparatus as set forth in claim 11 wherein the means for
detecting includes a laser heterodyne interferometer means.
13. Apparatus as set forth in claim 11 wherein the tip has at least
two characteristic resonant frequencies and wherein the radiation
is chopped at a frequency that is a function of a first one of the
characteristic resonant frequencies.
14. Apparatus as set forth in claim 13 wherein the translating
means includes means for oscillating the tip at a second one of the
characteristic resonant frequencies.
15. Apparatus as set forth in claim 11 and further comprising means
for cancelling spurious vibration of the sample due to bulk thermal
expansion of the sample, the means for cancelling comprising:
second means for illuminating the sample with radiation having a
second wavelength that differs from the characteristic
wavelength;
means for chopping the radiation having the second wavelength out
of phase with the radiation having the characteristic
wavelength.
16. Apparatus as set forth in claim 11 and further comprising means
for cancelling spurious vibration of the sample, the means for
cancelling including means for applying an a.c. voltage between the
tip and the sample.
Description
FIELD OF THE INVENTION
This invention relates generally to spectrographic method and
apparatus and, in particular, to an Atomic Force Microscope used
with a source of radiation selected to be absorbed by atoms or
molecules to be investigated. A cantilevered vibrating tip of the
Atomic Force Microscope is scanned over an illuminated sample
surface and directly detects and measures the resulting atomic or
molecular increase of size, thereby detecting both the presence and
location of the atoms or molecules under investigation.
BACKGROUND OF THE INVENTION
The term microscopy is employed where a surface is imaged with
radiation of a same energy. Where radiation of different or varying
energies is used the term spectroscopy is generally employed. Dual
purpose instruments are generally designated as microscopes even
when they perform spectroscopic investigation as well.
Spectroscopic analysis of surfaces at atomic scales is desirable
for a number of reasons, including the identification and
characterization of surface impurities in semiconductor,
superconductive and other structures.
In U.S. Pat. No. 4,343,993, Aug. 10, 1982, Binnig et al. describe a
vacuum electron tunneling effect that is utilized to form a
scanning tunneling microscope. In an ultra-high vacuum at cryogenic
temperature, a fine tip is raster scanned across the surface of a
conducting sample at a distance of a few Angstroms. The vertical
separation between the tip and sample surface is automatically
controlled so as to maintain constant a measured variable which is
proportional to the tunnel resistance, such as tunneling
current.
In a journal article entitled "Atomic Force Microscope", Physical
Review Letters, Vol. 56, No. 9, G. Binnig et al. at pages 930-933
described an atomic force microscope that is said to combine the
principles of the scanning tunneling microscope and a stylus
profilometer.
In U.S. Pat. No. 4,724,318, Feb. 9, 1988, Binnig describes an
atomic force microscope wherein a sharp point is brought near
enough to the surface of a sample to be investigated that forces
occurring between the atoms at the apex of the point and those at
the surface cause a spring-like cantilever to deflect. The
cantilever forms one electrode of a tunneling microscope, the other
electrode being a sharp tip. The deflection of the cantilever
provokes a variation of the tunnel current, the variation being
used to generate a correction signal which can be employed to
control the distance between the point and the sample. In certain
modes of operation, either the sample or the cantilever may be
excited to oscillate in a z-direction. If the oscillation is at the
resonance frequency of the cantilever, the resolution is
enhanced.
In U.S. Pat. No. 4,747,698 Wickramasinghe et al. describe a
scanning thermal profiler wherein a fine scanning tip is heated to
a steady state temperature at a location remote from the structure
to be investigated. Thereupon, the scanning tip is moved to a
position proximate to, but spaced from the structure. At the
proximate position, the temperature variation from the steady state
temperature is detected. The scanning tip is scanned across the
surface structure with the temperature variation maintained
constant. Piezo electric drivers move the scanning tip both
transversely of, and parallel to, the surface structure. Feedback
control assures the proper transverse positioning of the scanning
tip and voltages thereby generated replicate the surface structure
to be investigated.
In a journal article entitled "Atomic Force Microscope-Force
Mapping and Profiling on a Sub 100-A Scale", J. Appl. Phys. 61
(10), 15 May 1987, Y. Martin et al., at pages 4623-4729 describe a
technique for accurate measurement of the force between a tip and a
material, as a function of the spacing between the tip and the
material surface. The technique features a tip that is vibrated at
close proximity to a surface in conjunction with optical heterodyne
detection to accurately measure the vibration of the tip. The
technique enables the measurement of tip displacements over large
distances and over a wide range of frequencies, which is a major
advantage over the previous methods. The technique is applicable to
non-contact profiling of electronic components on scales varying
from tens of microns to a few tens of angstroms. A second
application is described wherein material sensing and surface
profiling are achieved simultaneously.
In a journal article entitled "High-resolution capacitance
measurement and potentiometry by force microscopy", Appl. Phys.
Lett. 52 (13), Y. Martin, D. W. Abraham and H. K. Wickramasinghe at
pages 1103-1105 describe an atomic force microscope employed for
potentiometry and for imaging surface dielectric properties through
the detection of electrostatic forces.
The electron tunneling effect is shown to be applicable to
spectroscopic analysis is a journal article entitled "Tunneling
Spectroscopy", B. J. Nelissen and H. van Kemper, Journal of
Molecular Structure, 173 (1988) at page 141-156. This article
describes the use of the Scanning Tunneling Microscope as a
spectroscopic probe. These authors note that spectroscopic methods
use energetic probes, usually photons, to gain desired information.
They further note that for spectroscopy in conducting solids the
use of photons is not an obvious choice, since the electrons inside
the solid can be used as spectroscopic probes.
In a journal article "Photothermal Modulation of the Gap Distance
in Scanning Tunneling Microscopy", Appl. Phys. Lett. 49 (3), 21
July 1986, by Nabil M. Amer, Andrew Skumanich and Dean Ripple at
pages 137-139 describe the use of the photothermal effect to
modulate the gap distance in a tunneling microscope. In this
approach, optical heating induces the expansion and buckling of a
laser-illuminated sample surface. The surface displacement can be
modulated over a wide frequency range, and the height (typically
.sctn. 1 Angstrom) can be varied by changing the illumination
intensity and modulation frequency. The method is said to provide
an alternative means for performing tunneling spectroscopy.
As is apparent the Scanning Tunneling Microscope (STM) and the
Atomic Force Microscope (AFM) have provided an efficient and
accurate means to perform the observation of atomic features on
surfaces However, such prior art techniques have not overcome the
problem of providing an efficient and accurate means to perform
spectroscopy on the atomic and/or molecular scale, although certain
attempts have yielded some limited results, namely voltage
spectroscopy in STM, "peak force detection" spectroscopy with the
AFM, temperature spectroscopy with the Thermal Profiler, and Auger
spectroscopy with a Field emission microscope.
It is thus an object of the invention to provide apparatus and
method for performing spectroscopy at atomic scales.
It is another object of the invention to provide method and
apparatus for practicing Atomic Photo-Absorption Force Microscopy
(APAFM) that beneficially combines both atomic resolution and
spectroscopy for use in wide range of analytical applications.
SUMMARY OF THE INVENTION
The foregoing problems are overcome and the objects of the
invention are realized by an Atomic Photo-Absorption Force
Microscope constructed and operated in accordance with the
invention. In accordance with the invention a radiation source has
a wavelength selected to be preferentially absorbed by atoms or
molecules associated with a sample surface under investigation.
Absorption of the radiation raises at least one outer shell
electron to a higher energy level, resulting in an increase in
radius of the atom or molecule. A tip coupled through a lever to an
Atomic Force Microscope is scanned over the surface and operates to
directly measure the resulting atomic or molecular increase of
size, thereby detecting both the presence and location of the atoms
or molecules under investigation. Operation in an a.c. mode by
chopping the incident radiation and measuring the corresponding
a.c. induced tip movement beneficially increases the sensitivity of
the technique, particularly if the a.c. frequency is chosen at a
resonance of the tip-lever combination.
In accordance with a method of the invention there is disclosed a
method of performing spectroscopy at atomic scales. The method
includes a step of illuminating a sample with radiation having a
characteristic wavelength selected for being absorbed by atoms or
molecules of interest such that at least one outer shell electron
is raised to a higher energy level, resulting in an increase in a
radius of the atom or molecule. The method includes a further step
of translating a probe tip proximal to the surface of the sample,
the probe tip being mounted such that it experiences a detectable
movement in response to being positioned near to an atom or
molecule of increased radius. The method also includes a step of
detecting the movement of the probe tip for indicating the presence
of the atoms or molecules of interest. The probe tip has at least
two characteristic resonant frequencies and the step of
illuminating includes a step of chopping the radiation at a
frequency substantially equal to a first one of the characteristic
resonant frequencies. Furthermore, the step of translating includes
a step of oscillating the probe tip perpendicularly to the surface
at a second one of the characteristic resonant frequencies.
BRIEF DESCRIPTION OF THE DRAWING
The above set forth and other features of the invention will be
made more apparent in the ensuing Detailed Description of the
Invention when read in conjunction with the attached Drawing,
wherein:
FIG. 1 is a block diagram, not drawn to scale, illustrating the
APAFM of the invention disposed relative to a sample; and
FIG. 2 is a diagram that illustrates APAFM tip displacement as a
function of atomic radius and illuminating radiation.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1 illustrates an APAFM 1 disposed relative to a sample. The
APAFM 1 includes an Atomic Force Microscope (AFM) 10 that is
similar in many operational characteristics to the AFM described in
the above mentioned journal article entitled "Atomic Force
Microscope-Force Mapping and Profiling on a Sub 100-A Scale", J.
Appl. Phys. 61 (10), 15 May 1987, Y. Martin et al., at pages
4623-4729. The AFM 10 is configured to operate in a repulsive force
mode. A tungsten tip 12 disposed at the end of a wire lever 14 is
mounted on a piezoelectric transducer 16. The transducer 16 is
driven by a source 16a of alternating current (a.c.) and vibrates
the tip along a z-axis at the resonant frequency of the wire lever
14, which acts as a cantilever. A laser heterodyne interferometer
18 accurately measures the amplitude of the a.c. vibration. The
tip/lever combination (12,14) is also coupled to suitable
piezoelectric transducers (not shown) for being translated along an
x axis and a y axis parallel to a surface 24a of a sample 24.
In an illustrative embodiment of the invention the lever/tip
(14,16) is a unitary body comprised of a tungsten rod, etched into
a cone, having a length of approximately 460 microns, a base
diameter of approximately 15 microns, and a final tip diameter of
approximately 0.1 micron. The last 40 microns of the cone are bent
at 90.degree.. The tip spring constant k, the first and the second
resonant frequency and the Q factor of the lever were determined to
be 7.5 N/m, 72 kHz, 200 kHz and 190, respectively.
It should be understood that these tip and other characteristics
are exemplary and are not to be construed in a limiting sense.
Also, it should be understood that while there is no intent to
limit the scope of the present invention by the theory presented
below, this theory is believed to be both accurate and consistent
with observable facts and accepted scientific principles.
The APAFM 1 further includes a radiation source 20 that provides
periodic, chopped radiation, indicated by the numeral 22, of
selected wavelength to illuminate the sample 24 disposed within the
vicinity of the tip 12. The radiation 22 is preferentially absorbed
at the sample 24 surface 24a by exciting electrons of atoms 26, or
molecules, to a higher energy state 28. One suitable radiation
source is a focussed and chopped tunable laser, such as a dye laser
or a frequency doubled dye laser. The required wavelength may vary
from the near infrared, which is required to probe molecular bonds,
up to ultraviolet which is required to excite electrons on low
atomic orbitals. An acoustooptical-type modulator can be employed
to chop the radiation beam. Typically the chopping frequency is
selected to coincide with the lowest resonant frequency of the
lever/tip (14,16), or 72 kHz for the embodiment discussed herein.
The absorption of the radiation 22 results in an increase in radius
of the atom or molecule from a first radius (r1) to a second larger
radius (r2).
In accordance with the invention the radiation source 20 has a
wavelength selected to be preferentially absorbed by atoms or
molecules of interest. The tip 12 is scanned parallel to and
proximal to the surface 24a and operates to directly measure the
resulting atomic or molecular increase of radius, thereby detecting
both the presence and location of the atoms or molecules under
investigation. Operation in an a.c. mode by chopping the incident
radiation 22 and measuring the corresponding a.c. induced tip 12
movement has been found to beneficially increase the sensitivity of
the technique, particularly if the a.c. frequency is chosen at a
resonance of the tip-lever combination.
The tip/lever combination (12,14) resonates at two frequencies
.omega..sub.0 and .omega..sub.1 and the AFM 10 detects both of
these frequencies. From the vibration at .omega..sub.1, generated
by the source 16a, the AFM 10 controls the spacing between tip 12
and sample 24 and displays the surface topography. The radiation 22
is chopped at .omega..sub.1 and the spectroscopy of surface 24a is
derived from the tip 12 vibration at .omega..sub.1.
The invention advantageously exploits the characteristic of atomic
structure that causes the radius of an electronic orbital of an
atom to increase roughly with n.sup.2, where n is the principal
quantum number of the orbital. According to Slater's orbitals, the
radius of an atomic orbital has a value n.sup.2 /(Z-s) in units of
Bohr radii, where Z is the atomic number and (s) is some screening
factor smaller than Z. It is apparent from this formula that the
size difference between two adjacent orbitals at the periphery of
an atom is of the order of one angstrom. In the example of Rb+
which has 36 electrons, the approximate radii for the orbitals 1s,
2s and 2p, 3s 3p and 3d, 4s and 4p are 0.1, 0.3, 0.9 and 3
Angstroms, respectively. The orbital of higher order (n=5) that
corresponds to an excited state of Rb+ exhibits a significantly
larger radius of approximately 6.0 Angstroms.
The "apparent" size of an atom or molecule is very similar to the
size of the external electronic orbital, as far as bonds with other
atoms or forces are concerned. Hence, exciting an atom by bringing
an electron to an orbital larger than the last normal orbital of
the atom significantly increase the apparent size of that atom, by
up to several Angstroms.
The lifetime of an excited atom depends widely on a number of
factors including radiative or non-radiative decay to the
fundamental state and coupling to the surrounding media. For a
radiative decay in the case of a strongly allowed electric-dipole
atomic transition in the optical frequency, as is exploited in the
APAFM 1, a value quoted by Siegman in "An Introduction to Lasers
and Masers", McGraw-Hill (1971) at page 100 is 10 nanoseconds (ns).
However, atoms in a crystal or solid can exhibit more rapid
non-radiative decays, down to picoseconds (ps), due to a strong
coupling of the internal atomic oscillations to the surrounding
crystal lattice. However, even in this case a few transitions of
selected atoms in solids are so decoupled from lattice vibrations
that they have relatively long lifetimes. For example, Nd.sup.3+ in
a has a 4 millisecond (ms) lifetime [Siegman, p. 101-2].
Although it may seem apparent that the lifetime of the excited
state is a dominant and important factor for successful operation
of the APAFM 1 such may not be true for several reasons.
Firstly, the tip 12 of the AFM 10 does not measure an average size
of the atom, but the "peak" (r2) size of the atom, as shown in FIG.
2. In FIG. 2, t1 is the average time interval between incident
photons and t2 is the life time of an excited atom or molecule. By
example, with a maximum radiation 22 flux of 1mW focused within a
one micron spot, corresponding to 10.sup.7 photons per second, the
duty cycle (t2/t1) of the atom in the excited state may be very
small, as depicted by the narrow peaks in the diagram of FIG. 2.
Due to the strong repulsive forces between the tip 12 and the atom,
and because the tip 12 cannot follow the fast transitions of the
atom, the tip is repelled from the atom to a distance 30 dictated
by radius (r2) of the excited state. The tip 12 maintains this
distance 30 during the on-time of the incident radiation 22 and
approaches the radius r1 of the orbital of the ground state
(distance 32) when the radiation 22 is off. The radiation chopping
frequency is thus preferably tuned to a resonance of the tip-lever
(12,14) to increase sensitivity.
Secondly, a consideration of energy is even more appropriate in
sensing atomic size variations with the tip 12. The transitions
that are considered are transitions from a high order orbital to a
non-populated external orbital, i.e. Balmer (from n=2), Paschen
(n=3) or Brackett (n=4) transitions. The associated photon energy
is typically a few electron volts (eV). This energy of the incident
photons is the energy that eventually moves the tip 12. It can be
shown that the energy required to move the tip 12 is the spring
energy: ##EQU1## For k=100N/m,x=1 Angstrom and Q=200 this required
spring energy is much smaller than the energy of a single photon.
Furthermore, several photons are contributing to the tip 12
displacement during each "on" alternate of the chopped radiation 22
frequency. Therefore, the presence of the tip 12 induces only a
small perturbation to the atom that is being excited.
Bulk thermal expansion of the sample due to the radiation
absorption may induce some spurious tip 12 vibration at the
tip-lever resonances. These spurious vibrations are minimized by
mounting the sample 24 on a transparent holder 34 so that radiation
absorption will occur only by those atoms whose transition is tuned
to the radiation wavelength. Additionally, thermal effects may be
suppressed by employing a second radiation source 36 having a
wavelength that differs from the wavelength of source 20. The
source 36 is chopped out of phase with the first radiation source
20 and heats the bulk of the sample 24 so that its temperature and
thermal expansion remain substantially constant in time. In
practice the amplitude, chopping frequency and phase of the source
36 may be varied or adjusted to obtain a desired degree of
cancellation of spurious oscillation due to thermal effects.
For an electrically conductive sample, or a thin sample disposed
upon an electrically conducting substrate, spurious vibration may
be cancelled by employing an electrostatic force. The electrostatic
force is generated by applying an a.c. voltage between tip 12 and
sample 24 in a manner disclosed in the aforementioned journal
article entitled "High-resolution capacitance measurement and
potentiometry by force microscopy", Appl. Phys. Lett. 52 (13), Y.
Martin, D. W. Abraham, H. K. Wickramasinghe. Preferably the a.c.
signal has a frequency that is approximately one half of the lower
resonant frequency, or 36 kHz for this embodiment, indicated by the
source 38 designated by .omega..sub.1/2.
The energy of the electronic transitions actually detected may
differ from the orbital energies tabulated by using classical
methods. That is, in the APAFM 1 the atoms being illuminated are
located at the surface 24a of the solid sample 24, as opposed to
atoms within the bulk of a material as are typically considered by
classical analytic technique. Thus, surface effects may influence
the energy levels of the atoms. The presence of the tip 12 may also
alter the energy levels, specially if the tip 12 is electrically
biased. However, these factors need not be considered in a negative
or limiting sense in that they allow new types of spectroscopy to
be done. That is, atomic scale spectroscopy is accomplished that
addresses surface atoms and that operates in accordance with
functions of local parameters such as electric field force and
surface effects.
The sensitivity of the APAFM 1, based on considerations of
radiation flux and of photon and tip energies, is suitable for use
with a number of materials including insulators, where tunneling
and inverse photo-emission spectroscopy are not feasible.
Thus, while the invention has been particularly shown and described
with respect to a preferred embodiment thereof, it will be
understood by those skilled in the art that changes in form and
details may be made therein without departing from the scope and
spirit of the invention.
* * * * *